Distortion compensator



April 16, 1929. PE O 1,709,037

DISTORTION COMPENSATOR Filed June 28, 1924 3 Sheets-Sheet 2 H y/my ,0aw g him mwb 3 g mm mm 2:29, 6 1% -4 V SE02- Time in seam ads INVENTOR ATTORNEY April 16, 1929. s. E. PERO 1,709,037

DIS'I'ORTION COMPENSATOR Filed June 28, 1924 5 Sheets-Sheet 5 l I l I l Al 0 500 1000 1500 2000 2500 5000 3500 4000 [frequency 0.2 F/zquflwy mm 0 w, )0 MINER? 10 0 w w 11 /2 INVENTOR :SZEfiGl JO BY 7AF ATTORNEY rrE 1,709,037 PATENT OFlCE.

SALJLIJE F. FEED, OF NEW YORK, N. Y., A$SIGNOR T AMERICAN TELEPHONE AND TELEGPH COMPANY,- it UUFPU'TIUN OF NEW Y'Ult.

DISTORTIUN GOEN'SAIILOF.

Application filed June %8, 19%. Serial No. $23,003.

An object of my invention is to provide a new and improved electrical transmission system in which the frequency components of the current within a certain desired frequency range will be in the same phase relation with one another at the receiving end as at the transmitting end. Another object of my invention is to make the received composite wave form in such a system of the same shape as the transmitted wave form. Another object is to provide for a desired displacement in time of the respective frequency components of a composite alternating current. Another object is to provide for a suitable phase shift of currents of different frequencies in a circuit so as to bring them into a desired phase relation. Another object is to provide for a relative phase shift of the frequency components in the output from a transmission line to compensate for the phase shift on the line and to restore the components at the receiving end of the line to the same phase relation as at the sending end;v Still another object is to provide a transducer at the receivin end of a transmission line that shall compensate the distortion by differential hase shift on the line. a In the following speclfication, I show several specific examples of practice according to my invention. It will be understood that the specification relates largely to these particular cases and that the invention is defined in the appended claims.

The term transducer employed in this specification is meant to refer to any apparatus having a pair of terminals for input electromotive force and another pair of ter minals for output electromotive force, the output being a function of the input. 5

Referring to the drawings, Figure 1 is a symbolic diagram of a system embodying my invention; Fig. 2 is a diagram showing phase displacement on a certain transmission line as a function of frequency; Fig. 3 is a diagram showing phase displacement as a function of frequency for certain sections of the same line and also showing the compensating phase displacements in appropriate networks that are to be combined with the line; Fig.

4 is a diagram showing the network represented by the symbol P in Fig. 1; Fig. 5 is a diagram showing the attenuation in the-same line compared with the attenuation in the combined line and network; Fig. 6 is a diagram showing the improvement of the arrlval curve by the use of my invention; Fig. 7 1s a diagram showing a modified network adapted for use in association with a certain loaded line; Fig. 8 is a diagram showing the phase dlstortion for the loaded line; Fig. 9 is a diagram showing the efiect of my lmproved network in compensating the distortion on this loaded line; Fig. 10 is a diagram for a detail modification; and Figs. 11 and 12 are diagrams that will be referred to in discussmg the principles involved in my invention.

Referring to Fig. 1, the line L connects the transmitter T with the receiver R. This line L retards or delays the current components of different frequencies by different amounts, and apparatus is interposed at the receiving end to compensate the differential retardation (as well as the differential attenuation) so that the received wave form shall be the same as at the transmitting end. N is a network whose impedance looking in the direction of the arrow is substantially the same as the characteristic impedance of the line L. The output conductors from the network N are connected in the grid circuit of a three-electrode'vacuum tube Q so that practically no current is drawn from the network N but an electromotive force is applied to the grid of the tube Q which is proportional to the current received in the network N.

The output circuit of the vacuum tube Q is connected with the input of the network P which is shown in some detail in Fig. 4. As will be explained, its effect is to compensate the phase distortion produced in the line L. The output from the network P (which miles, whose constants are resistance R0 =2.74 ohms per mile; 7 inductance L =0.001 henrys per mile;

capacity 0,, =0.296 microfarads per mile,

Also let it be assumed that compensation is desired over the frequency range from 2 to 25 cycles per second.

The curve of Fig. 2 gives the angle of retardation or delay for the various frequency components withm the range below 25 cycles per second. For example, the component current of frequency 15 cycles per second arrives at the network N in Fig. 1 more than 3 radians behind the input for that component current at the transmitting end of the line L.

The received current would be without distortion if, instead of the curve shown in Fig. 2, its curve were a straight line through the origin. This will be apparent because the phase anglewould then be proportional to the frequency, and the absolute retardation of all frequencies would be the same, so that the wave form at the receiving end would be unchanged from that at the transmitting end, so far as distortion due to phase displacement is concerned. To apply my invention as exemplified in the example under discussion, I draw the straight line shown in Fig. 2 to pass through the origin and to intersect the curve at the upper frequency limit, that is, at 25 cycles er second. Then I introduce a compensating network P such that the resultant graph of phase angle against frequency for the combined line and network is given by this straight line.

I make the network P in three sections, each of which compensates for 500 miles of the line. Aswill be shown presently, the phase shift produced by the initial 500 mile section 0 the line differs for all frequencies by a certain angle from the phase shift produced by each of the remaining 500 mile sections. This is shown in Fig. 3. The curve 1 gives the phase shift for the initial 500 mile section of the-line as a function of frequency and the curve 2 gives the phase shift for each of the remaining 500 mile sections. The or di-nates of these two curves at any frequency differ by an angle a. This is the phase angle of the characteristic impedance of the line and its value is it is independent of the len th of the line.

The straight line 1' an 2 in Fig. 3 are drawn through the origin to intersect the respective curves 1 and 2 at the points corresponding to the limiting frequency 25 cycles. The dotted curve 1" has negative ordinates each of the same magnitude as the difference between the corresponding ordinates for the curve 1 and the line 1. Similarly, the curve 2" corresponds to 2 and 2.

As shown in Fig. 4,1 provide a network of three sections, the first P and the two remaining sections alike, each P The section P is intended and designed to have its plot of phase angle against frequency in substantial colncldence with the curve 1" of Fig. 3, and each of the sections P is intended and designed to have its plot of phase angle against frequency substantially coinciding with the curve 2" in Fig. 3. Accordingly, P compensates for the initial 500 mile length of the line L and the two sections P compensate for the remaining 1,000 miles of the line L, so that the output of the network P in Fig. 1 has all its components in the same phase relation to each other as at the transmitter end of the line L. This output current from the network I? is then passed through the equalizer E, which compensates for differential attenuation to that point, so that the output from E has the same wave form in all respects as the input to the line L at the transmitter T. This output current is amplified at A and. received at The nature of the elements of the network of Fig. i and their relation to one another are shown clearly in the diagram. The letter 1" stands for the resistance in the three-electrode vacuum tube Q, of its plate circuit. The various resistance and reactance elements in the section P. and in each section P have the values given in the following table r+R =5000 ohms;

R =7690 ohms;

L 14-.2 henrys; 0, =2.85 mi; 1" +R' 5000 ohms;

1'? 51,100 ohms; L =26.0 henrys; 0', =1.56 inf.

The design equations by means of which R L and C, of the foregoing values are determined are R 2a/(T R2) 1 /a 4b a +R2 0 1 w w In Equatlons =25, and in general is the upper limiting frequency bounding the range within which distortion is to be compensated. It is at once apparent from Equations (1) that L0 so that the inductance-capacity combination is resonant at the frequency w... a and b of Equations (l) are determined by the following equations:

in general, in lflquations (2), 0' stands for the part of the ordinate of the corresponding curve 1 or 2 in Fig. 3 above the straight line 1 or 2, respectively. Thus 0" is the phase angle of distortion lag for the current of corresponding frequency in the section of the line under consideration. 0-, and 0' are values taken at particular frequencies between the lower and upper limits of the range within which compensation is to be effected. The network 1P, or P has enough undetermined parameters so that its curve 1 or 2 of Fig. 3 can be made to intersect the axis of abscissae at or near zero and at one other point, and in addition to pass through two assigned intermediate points, and a, and 0 are the ordinates of the curves 1 or 2, as the case may be, at arbitrarily assigned intermediate frequencies. in the particular example here under consideration, these two intermediate frequencies aredtfi cycles per second and 15 cycles per sec on lln general, the ws of Equations (2) are given by B wm mi (3) where in 2a 7 and ,f stands for the frequency.

to has already been explained and defined The as of Equations (2) are a and 00,, the particular values given by Equation when i has the particular values already mentioned corresponding to 0' and e 1 The crosses on or near curves 1" and 2 on Fig. 3 show the actual values of phase shift given by the network sections of Fig. l and show that the agreement is very close to that desired for perfect compensation as indicated by those curves.

lin Fig. 5 the attenuation on the line L as a function of frequency has been indicated'by a plot of the transfer voltage ratios; that is at any given frequency the ordinate gives the magnitude ratios of the output voltage to the input voltage. lit will be seen that for the full 1,500 mile length, the attenuation is very marked at higher frequencies, so that the transfer voltage has a very low value for thehigher frequencies. There is also a very noticeable falling OH in the value of this ratio comparing the voltage at the end of the first 500 mile section with the voltage at the in put. The dotted lines show the transfer voltages for the combined line and network, both. for the full 1,500 mile length of line and for the first 500 mile section. it will be seen that the transfer voltage decreases much less for the higher frequencies for the combined line and network than for the line alone. Accordingly it follows that while my improved network gives approximately complete compensation for phase distortion, it gives substantial partial compensation for attenuation distortion. Referring to Fig. 1, the network P corrects for phase distortion and partially for attenuation distortion, and the attenuation equalizer E completes the correction for attenuation distortion.

in Fig. 6 the arrival curves are shown for the 1,500 mile line alone and for the same line combined with the network. It will be seen that my improved network gives an improved arrival curve and thus improves the received signal shape and enables the line to be operated at higher signaling speed.

it have given a specific example of an en gineering application of the principle of my invention and have stated the particular de sign equations that if have found to be applica le for this example. it have also shown the manner of operation of the apparatus em bodying the invention in this particular iii-- stance. Further to exemplify my invention,

T will now show how to equalize for phase distortion in the case of a section of loaded line. Referring to i, let it be assumed that the line L is a loaded line 20 miles long, of i -gauge cable. Let the constants of the 20 mile loaded line be resistance R0 =8ILO ohms per mile;

inductance .lio =0.00l henry per mile;

capacity U0 =0062 microfarad per mile;

leakage G =20, X718 microml'ios for 900 cycles per second;

inductance of each loading coil =0.0l43 henry; capacity of each loading coil =0.00296 microfarad; distance between loading coils =1.136 miles.

in this case let it be assumed that compensation is desired over the frequency range from 0 to 4,000 cycles per second.

lit) The curve of Fig. 8 corresponds to that of Fig. 2 for the case of the submarine cable. lit

will be seen that whereas the angle of distorl 0 fl llli ' 800 and ments have the values given in the following table:

r+R, 5,000 ohms;

R 101,000 ohms; L 0.46 henrys 0 .0216 microfarads.

The design equations which determine the foregoing values are As before, the resistance R is chosen arbitrarily to make +R =5,000 ohms. In this case g =4,000. It is seen at once from the expressions for L and 0 that L0 so that tan in ten 0 (811132 (131 tall 0' x till]. 0' 7 $1 tan O'gitg tan 0'1 (2: 2 (00 tan 0 -:c tan a In general axis defined by the equation:

where 5-"; is the frequency. Two particular values are assigned for w; these are 800 and 3,000, and the substitution of these values in the foregoing equation gives 11:, and m respectively. c and 0' are the values in Fig. 8 .corresponding to the frequencies 27f g =3,000 respectively.

Thecrosses on Fig. 8 show the actual values of the phase shift given by the combined 20- -mile section of loaded line and thenetwork of'Fig. 7. Their proximity to the straight line through the origin shows the high degree of compensation effected by the network when combined with the line.

In Fig. 9, 0' is plotted in full lines against frequency with a larger unit for 0'. The dotted line plot gives theangle (with sign reversed) due to the network impedance.

If a more accurate approximation is desired than that given by the foregoing procedure, the network sections can be made as shown in Fig. 10 instead of as shown in Fig. 7. This gives more flexibility of design and enables us to fit the curve of the phase angle to the desired curve at two more points than as compared with Fig. 7 Thus referring to Fig. 9, with the design of Fig. 10 the curve (with sign reversed) may be made to go not as shown by the dotted line in Fig. 9, but more as shown by the solid line, that is above the axis at the extreme left and then below the axis.

In the preceding examples of Figs. 4 and 7, letting represent the angle of the transfer impedance of the phase corrector, p is given by the equation a2: yb-tan (7) where a and b have the values obtained by solving Equations (1) or (4). The phase correctors are designed so that b will approximate closely the negative of the angles a of Figs. 3 and 8. Thus the phase an 1e 41 of the form represented by Equation may be adapted to correct the phase distortion in various types of networks and transmission lines. a and bare positive constants determined by the arangement of the elements in the corrective network while a is a function of the applied frequency.- It may be required that this compensating phase angle given by Equation (7 shall have the value zero when the impressed frequency is zero, and also when it has some other value case put as ia/(1 Then when w=0 a =O and lI/ Q; when w=w w 00 and 41 0; and when 0 w 'w,,., then O rc o0 and 0. These conditions are realized in the circuit given for the loaded cable.

, but positive in the interval, for example,

tit

Eitt

intranet ment obtainahte in a singte section tr? a/t tiince it is assumed that h tt and acf tottotvs e an a;

co it;

11 ctaim:

t. In comhination a transmission t' l ducing various phase displacements ponent currents oi various treauencies, and. a netvvortr of successive sections connected with the tine, each section produ nhase disptacements the respective e uencies compensating tor a corresponding ot t it? tine, "whereby the received currei hhas its components the same phase retat on as the transmitted current i it. eonihination, a transmission tine producing various phase disptacements tor component currents ct vturious erpiencies netvvortr connected therevvi input impedance the same as the characteristic impedance oi"- the tine, and another netvvorh comprising three-etectrode vacuum tuhe having its grid circuit connected With said first nettvorh and adapted to produce compensating phase disptacements at the respective treguencies in a circuit associated with theptate side ct said tube, whereby the current output trom the second network has its components in the same phase retation as the current to the tine,

3. it netvvortr to compensate phase distortion on tine said network having a pair oi" input terminats and a pair of output terminats and comprising a three-etectrc de vacuum tube With its grid circuit connected to the in put terminate and with a pair oi parattet branches in the ptate circuit; one branch consistingott resistance and the other branch consisting out inductance and capacity, said ptate circuit atso having another resistance in a A it. netvvorh to compensate phase distortron on a hne, said netvvortr having a pair ct input terminate and a pair ct output terminats and consisting ot a pturatity ot sections connec md in seriea each section having a three-etectrode vacuum tube with its grid circuit connected to the input end of the section and having two parattet branches in its ptate circuit With resistance in one branch and in ductance and capacity in the other hranctn With the ptate circuit compteted by a high resistance across the grid circuit terminats oi the succeeding section 5. in. combination, a transducer Whose ptot oit phase retardation against frequency be tween tvvo trequencies is a singte vatued curve and in combination there-With a network Whose corresponding ptot is a comptemencurve, so that the sum of the two curves is a straight tine through the origin.

he in comhinatiom a transducer Whose ptot of phase distortion. against treduency is a singte vatued curve over a desired t'requency range tirom Zero to a particutar treouiency and in combination therewith a netvvorh Whose corresponding ptot is a complementary curve, so that the sum of the two curves is a straight tine through the origin intersecting the tirst mentioned curve at a point cor responding to the upper timiting trec uency ot' the said range. A

't. in combination, a transducer, a netivorh" and an attenuation equatiser the transducer retarding current components ditterentty and attenuating them differently according to their frequency, the netvvorh producing compensating retardations over a desired ire quency range, and the attenuation equatiaer producing compensating attenuations over the same range, whereby the components in the output trom the combination are retarded atitre and attenuated alike and have the same phase rotation and retative magnitude as the components in the input,

in testimony vvhereot, t have signed my name to this specification this 26th day at dune i I Phi Wt tit) 

